Grain threshing devices usually include a rotatable threshing cylinder operatively associated with stationary concaves that are normally opposite only a portion of the threshing cylinder. The separated grain passes through the concaves by the action of gravity. Additional threshing and separation may be accomplished by blowers or grain impact arrangements the operation of which is dependent upon the velocity and density differences between the threshed grain and the residual grain material. This type of apparatus, exemplified in U.S. Pat. No. 1,184,999, is of a generally low capacity due to the short time duration of the threshing and grain separating operations, the inability of the apparatus to efficiently handle different grain sizes under varying field conditions and the tendency of the grain residual materials to plug the separating mechanism so as to require frequent shut down time and operator inconvenience. Attempts to overcome some of these objections have not been entirely satisfactory.
Thus, in U.S. Pat. No. 2,811,158, the threshing mechanism is conventional but the mixture of separated grain and refuse material is introduced into a large perforated separating drum within which the mixture is rotated and advanced to provide an extended time period for grain separation axially of the drum, and through the peripheral wall thereof.
The grain separator of U.S. Pat. No. 3,425,423 has a threshing mechanism which functions to throw some of the threshed grain against an impact deflector assembly, concurrently with directing the remainder of the material into a revolving screened drum for final grain separating purposes.
With respect to the threshing apparatus of U.S. Pat. No. 2,053,148, a threshing cylinder is rotatable within a stationary separating drum, which is expandible transversely thereof to accommodate different kinds and conditions of grain. However, only the lower portion of the stationary drum is perforated for grain separating or screening purposes. In the degerminator apparatus of U.S. Pat. No. 2,108,655, there is disclosed a rotatable tapered cylinder axially movable relative to a coaxial stationary tapered cylinder to vary the concentric spacing therebetween to control the grinding action of the degerminator.